Tuesday, 25 October 2016

Microscopists in the 19th century were fascinated by frustules
[1], the patterned siliceous coatings of diatoms, varying from one species to
another (see below). When the algae die, frustules do not readily decompose and
they fall to the sea, or lake, bed, joining those from diatoms that live there.

Experts arranged the frustules of different species on
slides to produce kaleidoscopic patterns and this art form continues today in
the wonderful work of Klaus Kemp [2] and others. However, it wasn't just the
frustules that interested the many microscopists among the developing middle
classes, as living diatoms were also observed, including those that move by
gliding. This movement was described by Philip Henry Gosse [3] in one of his
books that popularised Natural History and microscopy;

In some cases, as in the genus Bacillaria,.. ..this movement of sliding
goes on till the frustules are on the point of separating, which then retrace
their course till such a catastrophe seems equally imminent in the opposite
direction.

We can see what Gosse means when we look at the video clip
above and, for an explanation of these movements, he refers us to a paper by
Wallich [4] who states:

The normal motion of the Diatomaceous
frustule is in two opposite directions, which accord with its longest diameter.
It is of a smooth, gliding nature, devoid of jerks or interruptions, and exhibits
itself at tolerably regular intervals. The rate at which it travels is not
uniform, being subject to variation on increase or diminution of light and
warmth. The rate is also materially influenced by the condition of the
endochrome, the motions being invariably more active and energetic when the
frustule is full.

Wallich links the gliding movement to "elongated
prehensile filaments", as particles are dragged along behind, or pushed in
front of, the moving diatoms. We now know that motile diatoms, of which there
are free-living as well as "colonial" types, move by exuding slime from
what is termed a raphe. The slime produces a force against the substratum, or
another diatom (should they be formed into a chain), that results in
propulsion. At first sight, this might seem an expensive process, but the slime
consists largely of carbohydrates that are generated in quantity during photosynthesis
and these chemicals become much expanded when mixed with water. By exuding concentrated
material through pores in the raphe, the rapid hydration then gives the
observed propulsive force. The accurate observation by Wallich [4] that
movement is associated with light and a "full" cell are evidence of
an excess of carbohydrate from photosynthesis that can then be readily exuded.

When we observe gliding diatoms, we focus on the organisms, as
we cannot see the slime that is produced, unless a chemical stain is used to
show its presence. Interestingly, slimes from diatoms and many other organisms,
from bacteria to mammals, have a most important role to play in the functioning
of ecosystems [5].

For Henry Gosse, the beauty of diatom frustules, and the
gliding movement shown by some of these algae, are further evidence of the
wonder of God's Creation. For those of us that believe in evolution, there is a
puzzle in thinking about how frustules developed through time and how the physiology
of the algae evolved, allowing an excess of carbohydrates to be used as
exudates that hydrate to produce a propulsive force. No-one would think that
the variety of frustules evolved to please us and our approach to the puzzle is
affected by our inability to understand the time over which evolution has occurred.
Also, we don't know whether mutations resulted in a series of small changes or
whether changes were on a larger scale. We can only speculate – and wonder.

Monday, 17 October 2016

We can all name several National Parks, and many
other designated conservation areas, in our various countries. It's a fair bet
that all of those named will be terrestrial, as marine conservation areas are
much less familiar to us – not surprising as many are rather difficult to
visit.

The Faroe-Shetland Sponge Belt MPA (Marine Protected Area)
[1] is, as its name suggests, a region of ocean bed rich in sponges and it also
has good populations of ocean quahogs (Arctica
islandica), a type of clam (see above) that is collected by dredging in
some areas. They are not overfished and may be sold as mahogany clams for
steaming, or used in the preparation of canned chowders and other products [2].
Like all clams, they have two shell valves that are held closed by the
contraction of adductor muscles and gape when the muscles are relaxed,
the valves being forced apart by the elastic hinge. When open, the animal feeds
by drawing in a current of water through an inhalant siphon and this passes over the gills before exiting through an exhalant siphon. In addition
to their use in respiration, the gills also trap particles and these are
conveyed to the mouth in mucus-bound packages. Particles identified as being of
food value are ingested while those that are not are rejected into the exhalant
current. Sorting occurs on organs called labial palps and the whole collecting
+ sorting mechanism is sophisticated and driven by tiny cilia on the surface of
the gill and on the palps. The millions of cilia on the gill create
the respiratory currents.

There are male and female clams and reproduction occurs after
sperm and eggs are passed out in the currents of water passing from the
exhalant siphons. Fertilisation occurs in the water column and there is then a
larval stage that lasts between 30 and 60 days [3]. In this time the tiny
larvae feed on minute particles and swim by means of cilia on their body surface,
undergoing a transformation within their larval life to a different form that
has developing shell valves. The larva moves down in the water column and, if a
suitable location is found, it settles and develops into the form with which we
are most familiar.

Like most bivalves, ocean quahogs are sedentary and they
bury themselves just below the surface, with the siphons protruding. On
occasions they burrow down further and then stay, with the shell valves closed,
for between 1 and 24 days [3]. No-one can explain this behaviour and it is accompanied
by a depression of the rate of metabolism, a trait that enables some unrelated freshwater
bivalves to remain closed up for two years.

The fame of ocean quahogs comes in the age achieved by some
specimens. After measurement of the growth rings on the shell, one clam was
found to be > 400 years old [4]; a more detailed analysis extending this to
> 500 years [5]. This is of interest to those studying aging [6] and it has
been suggested that ocean quahogs may provide good models for the study of the
aging of tissues in humans. We cannot resist being anthropocentric.

It is the life history of ocean quahogs that fascinates Natural Historians. The chances of fertilisation are good if sperm
come into contact with ova, but there is the risk that gametes, or young
embryos, might be drawn in by feeding currents of nearby quahogs and
become captured on their gills. Next come the risks of larval life and metamorphosis,
with predators of many kinds feeding on the larvae. If they survive, each must
then find a place to settle; by no means easy if the planktonic larva has been
carried on currents to areas that have unsuitable substrata. There is then the
challenge of sinking, and swimming, down through the water column and beginning
the process of turning into an adult. If there are many settlers, competition for available space will result and only once this is overcome can the growth
of the adult animal begin.

Ocean quahogs can thus live for seconds, for
hundreds of years, or any time in between these limits. When you have steamed clams, or
open a can of clam chowder, give a thought to the age of the animals that you
are eating and the miniscule chance they had of becoming adults and of
reproducing. It might also be that some of them were living on the sea bed
before you were born.

Wednesday, 5 October 2016

My annual "Coming up for Air" trip to Torbay [1] was in September this year. Fortunately, the weather was good, so I walked along the coast to re-visit places that were so important to me when I
was growing up. Returning home to land-locked Hertfordshire, I then read what Philip Henry Gosse wrote in Land and Sea [2] about the parts of
the Torbay shore that I had strolled through, and this made me appreciate Gosse's wonderful enthusiasm even more than usual.

I began by walking along the beach at Paignton and followed the
strand line, always an interesting place to a Natural Historian. Unfortunately,
there was little to see on this occasion and it was quite different to the
strand line after a storm, when masses of algae are washed up. Like Gosse, I had always enjoyed
exploring this evidence of the marine world of the bay, particular treasures being
blue-rayed limpets attached to straps of brown algae, now torn from
their anchorage. Of all the organisms I saw, I don't know why these limpets so appealed to me, but they are
certainly attractive to the human eye (see below).

In Land and Sea,
Gosse describes their appearance:

The shell is of unimpeachable
symmetry, polish, and delicacy; it is of a translucent horn-colour, and its
summit is marked with three fine lines of the most brilliantly-gemmeous azure.

Not all the creatures washed up are so obviously attractive and,
even fifty years ago, there was also much evidence of human pollution, with tar
balls and pieces of net being common. Flotsam and jetsam are a rich source of
natural materials, like wood, to the avid beachcomber, but there is now much more plastic refuse on beaches, as we continue to regard the sea as a convenient dumping place. I am impressed that artists like Jo Sayer can turn these plastic objects into attractive works of art that tell stories about what we are doing to Nature [4].

After the sands, I walked over Roundham Head and then on to
Goodrington Sands (the sequence is shown in order in the photographs below; a
route that Gosse walked in the opposite direction [2]). There was no promenade at Goodrington in the mid-nineteenth
century and, while I took the promenade walk and cliff path
on this visit, as a boy I preferred to scramble over the rocks when the tide
was low, just as Gosse would have done.

Gosse has a vivid description of some fisherman he
encountered on Goodrington Sands:

..away across the heavy sands, in
which we sink at every step, away obliquely to the left, where another bold
headland, Roundham Head, breaks the sweep of the bay, and for the present shuts
out Torquay from our view.

There is our working ground, at
the foot of those red cliffs. We diverge a little from a straight line, and
approach the edge of the sands, in order to see what those two men are so busy
about, as they trudge along the water-line with stooping backs and downward
gaze. Oh! they are fishermen taking solens, or razor-fish, as they call them. Each
carries a light, narrow, but deep spade in his hand, and, as he marks a little
jet of clear water that spirts upward from a small hole in the sand, he rapidly
thrusts in his instrument, and adroitly jerks out his prey.. .. The man scarcely
deigns it a glance, thinks nought of its curious structure, cares only for the
halfpence it will bring him in the fish-market, jerks it into his basket, and
watches for the next jet of water with which the frightened and retiring
mollusc shall betray its place of retreat.

Razor clams (see below) are still a prized delicacy and they
are certainly very effective at burrowing. Being a Natural Historian, Gosse was
fascinated by this, but, as he pointed out, the clams were merely a commodity to
the fisherman.

It was not only the wonders of Natural History that inspired
Gosse, but the knowledge that all he saw was evidence of God. To him [2]:

..the inimitable, unapproachable, incomprehensible
impress of Deity is there. Augustine says, "The soul bending over the
things Thou hast made, and passing on to Thee who hast made them, there finds
its refreshment and true strength."

Thus would I desire to contemplate
the works of God, as bringing to my sense ever-fresh proofs of His
all-pervading care, of His wondrous skill and wisdom, of His glorious majesty
and power. Above all, they are the productions of the august Word: it is not that
they were made by One who is infinitely great, but far removed from me, so that
I can only reverently admire Him at an immeasurable distance. No; they are productions
of the mind and hand of the Word (John i 3)..

..Yet let me not be mistaken. The
study of the creatures could never teach me this. Notwithstanding all that they
eloquently declare of the eternal power and Godhead of the Creator, they are
ominously mute when I ask them how He will deal with me, a sinner.

All this comes as no surprise to those that know Henry
Gosse's work and life. However, my appreciation of him does not extend to the
religious views that made him such a devout Creationist. I find it baffling, and
it is one of the reasons why I have never read any of Gosse's books on
religious themes (listed in [3]). I admire his work in Natural History and enjoy
his company in my imagination during my nostalgic visits to Torbay, but why did
he have such a need to proselytise? Was he trying to convince himself?